This invention relates generally to the field of electrostatic loudspeakers.
Historically, electrostatic loudspeakers have been acknowledged for their excellent sound quality and lack of audible coloration associated with more conventional cone/enclosure type loudspeakers. Even so, the electrostatic loudspeaker, which has been under development since the 19th century, has never successfully represented more than a very small percentage of loudspeakers sold. There have been many attempts to solve some of the electrostatic loudspeaker limitations such as: capacitive reactance, overly high directivity, frequency response aberrations, voltage breakdown limits, and transformer design/interaction.
Most electrostatic loudspeakers fall into one of a few configurations. The most basic configuration is a large, full range device without crossovers which suffers from a lack of sound dispersion, particularly in the higher frequency ranges. A second type of system consists of separate electrostatic elements, each covering a different frequency range. A multiple element system utilizes a crossover network between a large diaphragm area transducer for low frequencies that progressively crosses over to smaller diaphragm areas for the higher frequencies. This system has greater cost and complexity and does not allow the use of the total combined diaphragm area available for low frequencies because the diaphragm is divided into separately sized areas.
Another group of electrostatic devices attempts to solve the directivity problem by using a plurality of separate stator sections with a multitude of resistors. In these devices, each resistor is connected to a different stator section in an attempt to manipulate the drive signal to different parts of the diaphragm in a frequency selective manner. One attempt is shown in British Patent No. 537,931 to Shorter, entitled xe2x80x9cImprovements in Electrostatic Loudspeakers.xe2x80x9d Shorter teaches the use of a plurality of external stator resistors arranged to drive each section of the stator. Different resistance values are combined with different bias resistor values causing varying bias levels in each section. This approach requires a construction with many connection points being created across the diaphragm and a plurality of individual stator sections gapped and insulated from each other. Using multiple sections only produces an approximate, step-wise result unless a very high number of sections are implemented which makes the complexity reach levels of impracticality far short of the number that would provide seamless transitions. Another version of this approach is shown in U.S. Pat. No. 2,631,196 to Janszen, entitled Electrostatic Loud Speaker. This configuration is also complex and more expensive to manufacture.
Another drawback with electrostatic loudspeakers using external resistors is that they suffer from increased capacitance and/or a loss of drive in the gaps between sections. Because the edges of each section must be fixed there is a loss of energy at those points as compared to a larger continuous diaphragm, and multiple diaphragm systems are more difficult to manufacture.
It has been recognized that it would be an improvement over the state of the art to provide a new method and apparatus for an electrostatic speaker that allows electronic audio signals to be applied to the stators in a more controlled and patterned way. More specifically, it would be valuable to be able to control the radiation patterns of the speaker by applying electronic audio signals to the stators in a controlled configuration.
One embodiment of the invention is an electrostatic loudspeaker which includes a high resistivity stator with electrode conductivity per square unit that decreases with distance from the connection point. An electrical contact area of predetermined size has a resistivity that relates to the highest frequency of interest. Outside the electrical contact area, an immediately adjacent area of high starting value, and then outwardly decreasing surface resistivity of the stators interacts with the capacitive load of the stator/diaphragm structure. The decreasing surface resistivity operates as a distributed network where the active acoustic output is attenuated in a predetermined manner with increased frequency at all points distant from the contact area. The apparent acoustic source size is reduced with increased frequency to maintain enhanced dispersion across the operating range of the loudspeaker system.
In another embodiment, an electrostatic loudspeaker with a high resistance stator electrode (greater than 10K Ohms/square), includes a low impedance (less than 1K Ohms/square) connection area such that the output is attenuated with increased frequency at all points distant from the connection area.
A variety of improvements are derived from these embodiments, such as, reduced breakdown voltage concentrations, directivity control, reduction of capacitive reactance, reduction of the effects of transformer leakage inductance, flatter frequency response, visual transparency and simplicity of construction.
These and other objects, features, advantages and alternative aspects of the present invention will become apparent to those skilled in the art from a consideration of the following detailed description taken in combination with the accompanying drawings.